The invention is based on an electronic ballast according to the preamble of claim 1. In particular, it concerns the control design for an input stage of the electronic ballast.
The document W096/03017 (Krummel) discloses an electronic ballast for the operation of lamps, which has a two-stage structure. An input stage is operated from the rectified mains voltage and provides a DC voltage. This DC voltage is used to operate an inverter, which generates a high-frequency (compared with the frequency of the mains voltage) AC voltage for the operation of lamps. The input stage has the following tasks: For reducing mains current harmonics to regulation (IEC 1000-3-2) values, it has to correct the power factor on the mains side. It also has to provide a DC voltage which is substantially independent of mains voltage fluctuations.
In said document, the input stage is designed as a step-up converter. This very widespread solution has the advantage that it makes good with few components and achieves quite high efficiencies. However, the input stage with a step-up converter has the following disadvantages: The DC voltage which is delivered is always greater than the maximum of the mains voltage; it generates strong RF interference; the starting current is high and the control needs a converter which either functions in analog mode or, if it functions in digital mode, has a sampling rate which is substantially higher than the mains frequency. The last disadvantage has the following background: The step-up converter is a switched-mode converter. That is to say, it has a switch whose duty ratio significantly affects the input current. The term xe2x80x9cduty ratioxe2x80x9d generally denotes the ratio of the time during which the switch is on to the time during which the switch is off. In order to achieve a high power factor, the duty ratio needs to be adjusted to the mains voltage.
Less widespread is the use of a so-called SEPIC converter in the input stage. It is described in the following document:
R. Antonio et al.: xe2x80x9cEVALUATION OF BOOST, SEPIC AND CUK TOPOLOGIES AS POWER FACTOR CORRECTION STAGE IN ELECTRONIC BALLAST APPLICATIONSxe2x80x9d, IEEE Power Electronics Congress CIEP, Mexico, 1994. Although the SEPIC converter requires more components, the aforementioned disadvantages of the step-up converter can be avoided. In this case, the mode with a discontinuous current profile described in said document plays an important role: In this mode it is possible, with a constant duty ratio for the switches contained in the SEPIC converter, to achieve a power factor which leads to regulation mains current harmonics.
It is an object of the present invention to provide an electronic ballast according to the preamble of claim 1, which makes use of the advantages of the SEPIC converter in an inexpensive way.
This object is achieved, in the case of an electronic ballast having the features of the preambles of claim 1, by the features of the characterizing parts of claim 1. A particularly advantageous configuration is given in claim 2.
In order to control the output voltage and adjust the power factor of the SEPIC converter, an analog controller is used in said document. According to the invention, a digital controller is used to control the SEPIC converter. Further, the above-explained conditions for controlling the step-up converter which dominates the marketplace make it obvious that, when a digital controller is used, it must have a sampling procedure that can resolve the mains frequency. In this context, the term xe2x80x9canalog controllerxe2x80x9d is intended to mean a controller which reads in and processes actual values, and outputs control values, as a continuous function of time. A digital controller functions as a discrete function of time. The actual values are sampled only at discrete points in time, between which there are sampling intervals. A new control value is likewise output at discrete points in time. This gives rise to control intervals within which the control value is constant. For the step-up converter, it has been found that the sampling and control intervals need to be shorter than 1% of the period of the mains frequency, in order to meet the requirements of said applicable regulations.
The invention uses the aforementioned property of a SEPIC converter that, in the mode with a discontinuous current profile, a sufficiently good power factor can be achieved with a constant duty ratio. The duty ratio is not adjusted to the instantaneous mains voltage, but instead merely the output voltage of the SEPIC converter is controlled. The output voltage need not be sampled at intervals which are shorter than 1% of the period of the mains frequency. According to the invention, the control intervals, i.e. the time during which the duty ratio is kept constant, are now at least as long as 1% of the period of the mains frequency. For a mains frequency of 50 Hz, for example, a control interval that is at least 0.2 ms long is hence obtained.
A commercially available microcontroller constitutes a flexible solution for implementing the controller according to the invention for the input stage of the electronic ballast. Since the invention requires only long sampling intervals (compared with the control of a step-up converter), an inexpensive microcontroller can be selected.